docs: def Thom(x, rhol, rhog, mul, mug):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_ as given in [2]_.
  
      .. math::
          \alpha = \left[1 + \left(\frac{1-x}{x}\right)\left(\frac{\rho_g}
          {\rho_l}\right)^{0.89}\left(\frac{\mu_l}{\mu_g}\right)^{0.18}
          \right]^{-1}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      mul : float
          Viscosity of liquid [Pa*s]
      mug : float
          Viscosity of gas [Pa*s]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      Based on experimental data for boiling of water. [3]_ presents a slightly
      different model. However, its results are almost identical. A comparison can
      be found in the unit tests. Neither expression was found in [1]_ in a brief 
      review.
      
      Examples
      --------
      >>> Thom(.4, 800, 2.5, 1E-3, 1E-5)
      0.9801482164042417
  
      References
      ----------
      .. [1] Thom, J. R. S. "Prediction of Pressure Drop during Forced 
         Circulation Boiling of Water." International Journal of Heat and Mass 
         Transfer 7, no. 7 (July 1, 1964): 709-24. 
         doi:10.1016/0017-9310(64)90002-X.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      return (1 + (1-x)/x * (rhog/rhol)**0.89 * (mul/mug)**0.18)**-1